1. Field of Invention
Ammonia is a highly toxic gas that is usually stored under high pressure as a liquid. It presents substantial hazards in transportation, storage and handling. Ammonia is used in many industrial processes and as a reducing agent in selective catalytic reduction of nitrous oxide in many combustion processes. The Urea to Ammonia technology offers a safe means of producing ammonia on demand from urea, which is a safe non-toxic substance, to be utilized in processes, particularly selective catalytic reduction (SCR), in which a non-pure ammonia source can be utilized.
2. Brief Description of Related Technology
This invention relates to processes and systems for the removal of nitrogen oxides from combustion gas streams and for other industrial uses based on the hydrolysis of urea to ammonia.
One method for providing a pressurized gas stream for removing nitrogen oxides from a combustion gas stream by SNCR (Selective Non-Catalytic Reduction) or SCR (Selective Catalytic Reduction) processes includes feeding an aqueous solution of urea or mixtures of urea-containing biuret and/or ammonium carbamate into a reactor and hydrolyzing the urea therein at temperatures of at least 110° C. and under pressure, Cooper, et. al. U.S. Pat. No. 6,077,91, the disclosure of which is expressly incorporated herein by reference. The reaction rate is controlled by the input of heat to the reactor to produce a gaseous product stream containing ammonia and feeding it into a combustion gas stream at a controlled rate which is approximately the amount necessary to the demand for external use in removing nitrogen oxides. The rate of hydrolysis of urea must be balanced with the demand for ammonia to achieve thorough removal of nitrogen oxides while avoiding ammonia slip. In the prior art, this balance has been based on manipulation of the reactor pressure and heat input to the hydrolysis reactor in response to demand.
In the prior methods, typically there is a demand signal emitted by a detection system within the effluent combustion gas stream which is linked to the heat source and/or pressure controller at the hydrolysis reactor intended to maintain the ammonia-gas generation rate at a level equal to that required to scrub the nitrous oxides from the combustion gas. The urea solution level within the reactor is kept essentially constant by a fluid level detector governing the pumping rate of urea solution to the hydrolysis reactor. The solution level of the urea solution in the reactor is independent of the ammonia-gas requirement in the combustion gas stream. Since water is both a reactant in urea hydrolysis as well as being lost as steam in the ammonia going to the combustion gas stream, the effect of the feed rate of urea solution to the reactor being independent of ammonia demand while the urea solution level in the hydrolysis reactor remains essentially constant is that the concentration of water in the urea solution in the reactor varies. Variation in the water concentration causes fluctuation in the ammonia generation rate.
According to the present invention, the volume of aqueous urea-containing solution in the hydrolyzer varies. More particularly, the urea concentration in the aqueous solution in the hydrolyzer and the temperature are kept more nearly constant by the matching of the aqueous urea-containing solution feed rate to the hydrolyzer with the demand for ammonia-gas for scrubbing NOX, flue gas conditioning or other industrial purposes.
This invention is particularly useful for scrubbing NOX from combustion gas streams, in flue gas conditioning and other industrial uses which require relatively low rates of ammonia-containing product gas on the order of about 0.1 to about 100 pounds of ammonia per hour.